Trace element content of seagrasses in the Leschenault Estuary,Western Australia

Estuarine environments are particularly vulnerable to human impacts. In this study, trace elements in Ruppia megacarpa, Halophila ovalis, sediment and porewater were analysed to assess the potential contamination of the Leschenault Estuary, Western Australia, from a primarily agricultural drain. Sediment concentrations of Cd, Cu, Mn, and Ni and were highest nearest the drain while Al, As, Cr, Fe and Zn and were highest further from the drain. H. ovalis showed greater accumulation of Fe, Al, and As than R. megacarpa. Concentrations of Fe, Al, As, and Ni were generally higher in below-ground plant parts than above, suggesting uptake of these trace elements via the sediment-route pathway. This study suggested that the drain was a source of Cu and Mn, with these elements entering the estuary through water inflows. As and Fe, were highest furthest from the drain suggesting input of trace elements from sources other than the drain under study.     

Coupled study of water-stable isotopes and anions in porewater for characterizing aqueous transport through the Mesozoic sedimentary series in the eastern Paris Basin

This study presents data on chloride and bromide concentrations in porewater, water and anion diffusion coefficients, and their accessible porosities based on radial diffusion experiments on rock samples collected from a 2000 m-deep borehole (EST433) drilled by Andra in the eastern Paris Basin. The distributions of water stable isotope and chloride concentrations in porewater along this column reveal transient flows of water and solutes between the aquitard layers and the surrounding aquifers. These distributions confirm the occurrence of two separate aquitard/aquifer systems: “Oxfordian/Callovo–Oxfordian/Dogger” and “Dogger/Liassic/Rhaetian”. This separation is confirmed by Cl⁻/Br⁻ ratios, which are low in Liassic and Dogger porewater, suggesting the influence of primary brines, and which are close to the marine ratio in the Dogger groundwater. Based on these results, transport simulations in the two systems were carried out according to different scenarios. The simulation results confirm that transport properties obtained in the laboratory at sample scale may be extrapolated to the formation scale. It is highly probable that diffusion is the main transport process in the Callovo–Oxfordian formation. This may also be the case in the Dogger/Liassic layer, although a limited contribution from advection cannot be totally excluded. By testing different scenarios of boundary conditions in diffusive models, it is proposed that the Dogger aquifer was first activated in the Early Miocene.     

Tracer dispersal in sandy sediment porewater under enhanced physical forcing

This research (1) integrated a fluorescent dye injection and monitoring system for measuring the mixing of a fluorescent dye tracer (fluorescein) in permeable (sandy) sediments with a cabled ocean observatory, Kilo Nalu, Oahu, Hawaii, and (2) used this system to conduct remotely controlled in situ measurements of wave-enhanced porewater mixing in a physically well-characterized wave-dominated setting. Laboratory results indicated that the fiber-optic sensor is effective at measuring fluorescence-traced enhanced mixing in sandy sediments. Observed dye mixing, driven by paddle-generated surface waves in a laboratory wave channel was 2–3 orders of magnitude greater than molecular diffusion, and decreased with depth in the sediments. Similarly, field experiments used a remotely controlled submersible syringe pump for fluorescent tracer injection into sediment that was monitored with a fiber-optic sensor. These experiments were carried out at 10 m water depth, with mean wave heights of 0.3–1.5 m and periods of ∼15 s. At 15 cm sediment depth, transport rates of 0–5 cm h⁻¹ were measured, with maximum dispersion coefficients 2–3 orders of magnitude faster than sedimentary molecular diffusion. Hydrodynamic measurements collected simultaneously via Kilo Nalu correlated with porewater transport, with significant wave height and diffusion having the strongest covariation.     

Porewater chemistry in compacted bentonite: Application to the engineered buffer barrier at the Olkiluoto site

Compacted bentonite is used as sealing and buffer material in engineered barrier systems (EBS) of high-level radioactive waste repositories. The chemical characteristics of this clay and its porewater affect the migration of radionuclides eventually released from the waste. They also determine the integrity and long-term performance of the clay barriers. Key features are the structural negative charge and the large proportion of structural (interlayer) water of the main mineral montmorillonite, which leads to exclusion of anions and a surplus of cations in a large part of the porosity space. The objective of this contribution was to assess the impact of different porosity model concepts on porewater chemistry in compacted bentonite in the context of the planned Finnish spent nuclear fuel repository at Olkiluoto. First, a structural model based on well-established crystallographic and electrostatic considerations was set up to estimate the fractions of the different porosity types. In view of the uncertainty related to the chemical properties of the interlayer water, two very different model concepts (anion-free interlayer, Donnan space), together with a well-established thermodynamic model for bentonite, were applied to derive the porewater composition of the bentonite buffer at Olkiluoto. The simulations indicate very similar results in the “free” water composition for the two models and thus support the validity of the reference porewater concept commonly used in performance assessment of waste repositories. Differences between the models are evident in the composition of the water affected by the surface charge (i.e. diffuse double layer and interlayer). These reflect the conceptual uncertainty in current multi-porosity diffusion models.     

The estuarine chemistry and isotope systematics of U in the Amazon and Fly Rivers

Natural concentrations of ²³⁸U and δ²³⁴U values were determined in estuarine surface waters and pore waters of the Amazon and Fly (Papua New Guinea) Rivers to investigate U transport phenomena across river-dominated land–sea margins. Discharge from large, tropical rivers is a major source of dissolved and solid materials transported to the oceans, and are important in defining not only oceanic mass budgets, but also terrestrial weathering rates.On the Amazon shelf, salinity-property plots of dissolved organic carbon, pH and total suspended matter revealed two vastly contrasting water masses that were energetically mixed. In this mixing zone, the distribution of uranium was highly non-conservative and exhibited extensive removal from the water column. Uranium removal was most pronounced within a salinity range of 0–16.6, and likely the result of scavenging and flocculation reactions with inorganic (i.e., Fe/Mn oxides) and organic colloids/particles. Removal of uranium may also be closely coupled to exchange and resuspension processes at the sediment/water interface. An inner-shelf pore water profile indicated the following diagenetic processes: extensive (1 m) zones of Fe(III)—and, to a lesser degree, Mn(IV)—reduction in the absence of significant S(II) concentrations appeared to facilitate the formation of various authigenic minerals (e.g., siderite, rhodocrosite and uraninite). The pore water dissolved ²³⁸U profile co-varied closely with Mn(II). Isotopic variations as evidenced in δ²³⁴U pore waters values from this site revealed information on the origin and history of particulate uranium. Only after a depth of about 1 m did the δ²³⁴U value approach unity (secular equilibrium), denoting a residual lattice bound uranium complex that is likely an upper-drainage basin weathering product. This suggests that the enriched δ²³⁴U values represent a riverine surface complexation product that is actively involved in Mn–Fe diagenetic cycles and surface complexation reactions.In the Fly River estuary, ²³⁸U appears to exhibit a reasonably conservative distribution as a function of salinity. The absence of observed U removal does not necessarily imply non-reactivity, but instead may record an integration of concurrent U removal and release processes. There is not a linear correlation between δ²³⁴U vs. 1/²³⁸U that would imply simple two component mixing. It is likely that resuspension of bottom sediments, prolonged residence times in the lower reaches of the Fly River, and energetic particle–colloid interactions contribute to the observed estuarine U distribution. The supply of uranium discharged from humid, tropical river systems to the sea appears to be foremost influenced by particle/water interactions that are ultimately governed by the particular physiographic and hydrologic characteristics of an estuary.     

Water mass ages of coastal ponds estimated using Ra and Ra as tracers

Measurements of the naturally occurring radioisotopes ²²³Ra (t_1/2 = 11.4 days) and ²²⁴Ra (t_1/2 = 3.66 days) in southern Rhode Island salt ponds were combined with a simple model to obtain independent estimates of the age of these coastal waters. Surface water and porewater samples were collected quarterly in Winnapaug, Quonochontaug, Ninigret, Green Hill, and Pt. Judith-Potter Ponds, as well as nearly monthly in the surface water of Rhode Island Sound, beginning January 2002 through August 2003. Surface water activities ranged from 1–78 dpm 100 L^− 1 and 5–885 dpm 100 L^− 1 for ²²³Ra and ²²⁴Ra, respectively. Porewater radium activities ranged from 3 to 715 dpm 100 L^− 1 for ²²³Ra, and 57–4926 dpm 100 L^− 1 for ²²⁴Ra. Results indicate seasonally varying water mass ages for Ninigret (5–12 days), Winnapaug (2–6 days) and Pt. Judith-Potter Ponds (1–9 days) and, in contrast, relatively constant ages for Green Hill (5–7 days) and Quonochontaug Ponds (3–6 days).     

Chemical stratigraphy of recent sediments from a depth gradient in a meromictic lake, Nordbytjernet, SE Norway, in relation to variable external loading and sedimentary fluxes

Cores of recent sediments were sampled along a depth gradient in a 23 m deep kettle lake with stagnant deep waters containing exceptionally high concentrations of dissolved iron and manganese. Sediment cores were taken on two occasions, in 1978 and 1997, before and after an incidence of full circulation. The aims of this study are to see how oxic and anoxic conditions in the water column influence stratigraphy and sediment focusing and, to compare cores from 1979 and 1998 to see how measured element fluxes and external events are reflected in the chemical stratigraphy. Element analyses show characteristic stratigraphic patterns that depend on the ability to undergo redox transformations, sorptive properties and chemical equilibria in the anoxic deep waters and porewaters. In sediments from the oxic part of the lake Al, Cr, Co, Ni, Zn Cu, Cd, and Pb were well correlated. Positive correlations are seen between elements associated with primary production and sulphur. In the anoxic part of the lake most metals were positively correlated with carbonate. Phosphorus correlated positively with iron in sediments from oxic waters and negatively with manganese and iron deep-water sediments. Porewater analyses indicate that recycling from the deep-water sediments was negligible. The stratigraphy of lead agrees with the historic variation in atmospheric input and is used as a chronological marker. Assessed deposition rates agree with measurements in sediment traps. Most elements more than double their rates of deposition towards the deepest point of the lake, while sulphur, manganese and carbonate had maxima around the depth of the redoxcline in the water. Variations in the external loading and variable redox conditions in the deep waters explain variations in the chemical composition of recent sediments.     

Stable isotope evidence for multiple fluid regimes during carbonate cementation of the Upper Tertiary Hazeva Formation, Dead Sea Graben, southern Israel

Stable carbon- and oxygen-isotope compositions of calcite and dolomite cements have been used to understand porewater evolution in the Upper Tertiary Hazeva Formation within the Dead Sea Graben, southern Israel. Sandstone samples were obtained from four boreholes in three tectonic blocks of the graben over depths of 253–6448 m, a variation that largely reflects differential subsidence of individual fault-bounded blocks. Early carbonate cements dominate diagenesis. Calcite occurs at <1600 m, but was replaced by dolomite at greater depths. Dolomite at 1600–2700 m is Fe-poor (<0.8 mol% FeCO₃), and at 4700–6200 m, Fe-rich (0.5–7.2 mol% FeCO₃). Magnesite, anhydrite and halite are the final diagenetic phases. Calcite has positively correlated δ¹⁸O (+21‰ to +25‰) and δ¹³C (−6‰ to −2‰) values that generally decrease with depth. Dolomite has a wider variation in δ¹⁸O (+18‰ to +30‰) and δ¹³C (−8‰ to −1‰) values, which also generally are lower with increasing depth. However, the δ¹³C and δ¹⁸O values of dolomite from the uppermost 400 m of the Hazeva Formation in the Sedom Deep-1 borehole are anomalous in spanning the entire range of stable carbon and oxygen isotopic compositions over this relatively small interval.The decreasing dolomite δ¹³C values likely indicate an increased contribution of carbon from organic sources with increasing depth. Except for the uppermost 400 m, Hazeva Formation dolomite in the Sedom Deep-1 borehole has stable carbon-isotope compositions that imply initial dolomitization at much shallower levels, prior to the preferential subsidence of this tectonic block. The oxygen isotopic compositions of the calcite cement are best explained by equilibration at present burial temperatures (≤55 °C) with porewater of meteoric origin. Its δ¹⁸O values increased from −5‰ at the shallowest depths to 0‰ at 1600 m. The dolomite oxygen isotopic compositions also reflect equilibration at present burial temperatures with porewaters ranging from 0‰ at 1600 m to +7‰ at 3600 m (100 °C). In the deepest fault block (Sedom Deep-1 borehole), however, increasingly Fe-rich dolomite has (re)equilibrated with porewater whose δ¹⁸O values decreased from +9‰ at 4750 m (120 °C) to +1‰ to +2‰ by 6200 m (150 °C).Much of the dolomite likely formed at relatively shallow depths from saline brines derived from precursors to the Dead Sea. These infiltrated the Hazeva Formation, mixing with and largely displacing meteoric water, and dolomitizing calcite. Rock–water ratios tended to be high during these processes. However, the upper 400 m of the Hazeva Formation in the deepest fault block were likely deposited during its rapid tectonic subsidence, and largely escaped the initial style of dolomitization pervasive elsewhere in the study area. These sediments were also capped by evaporites. This relatively thin interval likely became a preferential conduit for brines that escaped underlying and overlying strata, including the Fe-rich, lower ¹⁸O fluids (evolved seawater?) present in the deepest part of the graben. These rocks present the most promising target for the passage and accumulation of hydrocarbons in the study area.     

CPT site characterization for seismic hazards in the New Madrid seismic zone

A series of cone penetration tests (CPTs) were conducted in the vicinity of the New Madrid seismic zone in central USA for quantifying seismic hazards, obtaining geotechnical soil properties, and conducting studies at liquefaction sites related to the 1811–1812 and prehistoric New Madrid earthquakes. The seismic piezocone provides four independent measurements for delineating the stratigraphy, liquefaction potential, and site amplification parameters. At the same location, two independent assessments of soil liquefaction susceptibility can be made using both the normalized tip resistance (q_c1N) and shear wave velocity (V_s1). In lieu of traditional deterministic approaches, the CPT data can be processed using probability curves to assess the level and likelihood of future liquefaction occurrence.     

Geochemical insights into contribution of petroleum hydrocarbons to the formation of hydrates in the Taixinan Basin,the South China Sea

Methane hydrate in the South China Sea(SCS)has extensively been considered to be biogenic on the basis of itsδ13C and δD values.Although previous efforts have greatly been made,the contribution of thermogenic oil/gas has still been underestimated.In this study,biomarkers and porewater geochemical parameters in hydrate-free and hydrate-bearing sediments in the Taixinan Basin,the SCS have been measured for evaluating the contribu-tion of petroleum hydrocarbons to the formation of hydrate deposits via a comparative study of their source inputs of organic matters,environmental conditions,and microbial activities.The results reveal the occurrence of C14-C16 branched saturated fatty acids(bSFAs)with relatively high concentrations from sulfate-reducing bacteria(SRBs)in hydrate-bearing sediments in comparison with hydrate-free sediments,which is in accord with the positive δ13C values of dissolved inorganic carbon(DIC),increasing methane concentrations,decreasing alka-linity,and concentration fluctuation of ions(Cl-,Br,SO2-,Ca2+,and Mg2+).These data indicate the relatively active microbial activities in hydrate-bearing sediments and coincident variations of environmental conditions.Carbon isotope compositions of bSFAs(-34.0％o to-21.2％o),n-alkanes(-34.5％o to-29.3％o),and methane(-70.7％o to-69.9％o)jointly demonstrate that SRBs might thrive on a different type of organic carbon rather than methane.Combining with numerous gas/oil reservoirs and hydrocarbon migration channels in the SCS,the occurrence of unresolved complex mixtures(UCMs),odd-even predominance(OEP)values(about 1.0),and biomarker patterns suggest that petroleum hydrocarbons from deep oil/gas reservoirs are the most probable carbon source.Our new results provide significant evidence that the deep oil/gas reservoirs may make a contribution to the formation of methane hydrate deposits in the SCS.